Add: [Video] move GameLoop into its own thread

This allows drawing to happen while the GameLoop is doing an
iteration too.

Sadly, not much drawing currently can be done while the GameLoop
is running, as for example PollEvent() or UpdateWindows() can
influence the game-state. As such, they first need to acquire a
lock on the game-state before they can be called.

Currently, the main advantage is the time spend in Paint(), which
for non-OpenGL drivers can be a few milliseconds. For OpenGL this
is more like 0.05 milliseconds; in these instances this change
doesn't add any benefits for now.

This is an alternative to the former "draw-thread", which moved
the drawing in a thread for some OSes. It has similar performance
gain as this does, although this implementation allows for more
finer control over what suffers when the GameLoop takes too
long: drawing or the next GameLoop. For now they both suffer
equally.

1 files changed, 32 insertions, 0 deletions

diff --git a/src/video/video_driver.hpp b/src/video/video_driver.hpp
index cfb853dba..423e46436 100644
--- a/src/video/video_driver.hpp
+++ b/src/video/video_driver.hpp
@@ -16,7 +16,11 @@
 #include "../gfx_func.h"
 #include "../settings_type.h"
 #include "../zoom_type.h"
+#include <atomic>
 #include <chrono>
+#include <condition_variable>
+#include <mutex>
+#include <thread>
 #include <vector>
 
 extern std::string _ini_videodriver;
@@ -31,6 +35,8 @@ class VideoDriver : public Driver {
 	const uint DEFAULT_WINDOW_HEIGHT = 480u; ///< Default window height.
 
 public:
+	VideoDriver() : is_game_threaded(true) {}
+
 	/**
 	 * Mark a particular area dirty.
 	 * @param left   The left most line of the dirty area.
@@ -84,6 +90,11 @@ public:
 	}
 
 	/**
+	 * Populate all sprites in cache.
+	 */
+	virtual void PopulateSystemSprites() {}
+
+	/**
 	 * Clear all cached sprites.
 	 */
 	virtual void ClearSystemSprites() {}
@@ -240,6 +251,16 @@ protected:
 	virtual bool PollEvent() { return false; };
 
 	/**
+	 * Start the loop for game-tick.
+	 */
+	void StartGameThread();
+
+	/**
+	 * Stop the loop for the game-tick. This can still tick at most one time before truly shutting down.
+	 */
+	void StopGameThread();
+
+	/**
 	 * Give the video-driver a tick.
 	 * It will process any potential game-tick and/or draw-tick, and/or any
 	 * other video-driver related event.
@@ -271,6 +292,17 @@ protected:
 
 	bool fast_forward_key_pressed; ///< The fast-forward key is being pressed.
 	bool fast_forward_via_key; ///< The fast-forward was enabled by key press.
+
+	bool is_game_threaded;
+	std::thread game_thread;
+	std::mutex game_state_mutex;
+	std::mutex game_thread_wait_mutex;
+
+	static void GameThreadThunk(VideoDriver *drv);
+
+private:
+	void GameLoop();
+	void GameThread();
 };
 
 #endif /* VIDEO_VIDEO_DRIVER_HPP */